1. Field of the Invention
The present invention relates to a quartz crystal unit constructed by holding a quartz blank with supporters, and more particularly to a quartz crystal unit with favorable frequency stability characteristics in response to changes in temperature and long-term aging by avoiding stress applied to the quartz blank with the supporter.
2. Description of the Prior Art
A quartz crystal unit is widely used in various electronic equipment including communication devices as a reference source for frequency and time. The quartz crystal unit is formed to include a quartz blank in a container, and one of such elements is a quartz crystal unit which holds a quartz blank with a pair of supporters for providing high stability.
FIGS. 1 and 2 are diagrams showing a configuration of a conventional quartz crystal unit. FIG. 1 is a vertical section view of the quartz crystal unit, while FIG. 2 is a plan view showing the quartz crystal unit from which a metallic cover of a container is removed.
The quartz crystal unit is configured to hermetically seal quartz blank 4 held by a pair of supporters 3a, 3b in a container comprising metallic base 1 and metallic cover 2. A pair of lead wires 6a, 6b pass through metallic base 1. Glass 5 is formed for sealing at the portions where lead wires 6a, 6b pass through base 1 to maintain electrical insulation between lead wires 6a, 6b and metallic base 1. Lead wires 6a, 6b are also referred to as hermetic terminals. Supporters 3a, 3b are formed as metallic members in L-shape, and the horizontal portions of supporters 3a, 3b are spot welded to the ends of lead wires 6a, 6b, respectively, such that the vertical portions thereof are opposite to each other.
Quartz blank 4 is an AT cut type in discoid shape, and as shown in FIG. 3, provided with excitation electrodes 7a, 7b formed on both major surfaces, respectively, and leading electrodes 8a, 8b extending from excitation electrodes 7a, 7b to the end surfaces at the opposite points on the periphery. Leading electrodes 8a, 8b are extended to the ends of quartz blank 4 and then folded back to the opposite surface for a predetermined length.
In FIG. 3, three axes, i.e., X, Y' and Z' axes are depicted. The X axis is crystallographically determined in quartz and exhibits the largest stress sensitivity, while the Y' axis indicates a direction perpendicular to the plane on which the quartz blank is cut with the AT cut. The direction in which leading electrodes 8a, 8b extend is inclined approximately 30 degrees toward the Z' axis. In this direction, the stress sensitivity in quartz blank 4 exhibits the smallest value. In other words, the quartz blank 4 is most insensitive against stress in this direction.
The end surfaces of leading electrodes 8a, 8b at the opposite points on the periphery of quartz blank 4 are abutted against the vertical portions of supporters 3a, 3b, and electrically and mechanically connected thereto with thermo-compression bonding, thereby holding quartz blank 4 by supporters 3a, 3b such that the major surfaces of quartz blank 4 is horizontally disposed with respect to metallic base 1. The vertical portions of supporters 3a, 3b are covered with metallic foils made from eutectic alloy of gold (Au) and germanium (Ge), not shown.
In such a quartz crystal unit, since the quartz blank is held at the opposite points on the periphery on the axis on which the stress sensitivity is least, it is possible to reduce changes in frequency in response to stress from the opposite points on the periphery. In addition, since the end surfaces at the opposite points on the periphery are held, favorable oscillation characteristics can be achieved without inhibiting oscillation displacements of the major surfaces. For these reasons, a configuration of this type is often employed especially in a high-stability quartz crystal unit which is accommodated and driven in a thermostatic bath for making frequency-temperature characteristics flat, for example.
In the quartz crystal unit with the aforementioned configuration, however, the supporting points for supporters 3a, 3b to metallic base 1, i.e. the positions where the aforementioned spot welding is performed, and the holding points for quartz blank 4 by supporters 3a, 3b are placed on the same straight line when viewed from above quartz blank 4, and the line passes through the central portion of quartz blank 4 which is a principal oscillation area. Thus, the axes on which supporters 3a, 3b expand and contract with changes in temperature and over time lie on the same line, and expansion and contraction forces locally concentrate to apply stress to quartz blank 4, causing variations in oscillation frequency of the quartz crystal unit.
Particularly, while a high-stability quartz crystal unit requires frequency stability on the order of ppb (10.sup.-9), the use of the aforementioned conventional configuration may lead to insufficient characteristics in terms of changes over time. When the opposite points on the periphery where the stress sensitivity is least are held, the portions of supporters 3a, 3b abutting against quartz blank 4 have a certain planar area, that is, the opposite points where the sensitivity is least are not held in spots. This is the reason the aforementioned configuration causes frequency changes due to stress.